U.S. Pat. No. 4,578,658 describes a microwave resonator and a microwave waveguide for conducting microwaves with high energy density. The components consist of a support body with a multi-layer coating of a thickness of about 5 .mu.m of copper and silver.
In the sintering of metallic/ceramic composite materials such as hard metals or cermet in a microwave oven it is necessary to heat the material to be sintered to temperatures of 1800.degree. C. which requires thermal insulation to avoid heat losses. There are basically two ways to do this, namely to insulate the charge being sintered with a microwave-transparent insulating material inside the resonator or to externally insulate the resonator housing.
The insulating material used inside the resonator housing that mainly consists of extremely pure aluminum-oxide foamed mats is not only very expensive but has the disadvantage that for example when sintering hard metals vapors sublime in the insulation and reduce the microwave transparency of the foam mats. The result is a "burning through" of the insulation as a result of it absorbing microwaves.
Applying external thermal insulation to the resonator housing, that does not need to be transparent to microwaves, has the disadvantage that all the parts inside the resonator housing, that is the resonator walls themselves, the coupling waveguide, any sensors or the like, must be made of a heat- and gas-resistant material to which end according to the state of the art temperature resistant metals are employed as materials for the microwave parts that can be exposed both to the microwave radiation as well as to the operating temperatures. Metals that are very resistant to high temperatures, such as for example molybdenum or tungsten, are not suitable as materials for microwave components since such metals carburize readily in the carbon in the sintering atmosphere. Graphite as well as other generally known materials resistant to high temperatures and elements of the sintering atmosphere are however much worse electrical conductors than metals so that they produce considerable losses in microwave energy. A graphite resonator housing thus is of low resonator efficiency.